Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1991-12-27
1993-10-12
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
G01R 3320
Patent
active
052529231
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to NMR imaging methods and has particular application in methods of obtaining NMR imaging information from solid objects.
2. Description of the Related Art
The NMR imaging of solid materials has not been developed to the same level of sophistication as the NMR imaging of liquid and quasi-liquid materials. The principal difficulties with NMR imaging in solids arise because of the rapid rate of decay of the transverse magnetization. In other words, the characteristic spin-spin relaxation time, usually designated T.sub.2, is short. To produce sufficient spatial localization of the selected nuclear spins in an object, large magnetic field gradients must be superimposed, which increases the rate of signal decay even further. In such circumstances much of the signal is lost in the dead time of the apparatus following the application of an rf excitation pulse. Also, the excitation pulse itself needs to be of large amplitude to excite resonance. Furthermore, because in solids the characteristic spin-lattice relaxation time T.sub.1 is much longer than T.sub.2, a considerable time interval of several T.sub.1 s, must elapse before an experiment can be repeated.
SUMMARY OF THE INVENTION
It is an object of the invention, to provide a method in which the above difficulties are overcome.
According to the invention a method of obtaining NMR imaging information from a solid object comprises subjecting an object to a static magnetic field, applying a 90.degree. rf excitation pulse on the object in the presence of a sinusoidally varying magnetic gradient field so that gradient echoes of the free induction signal are formed by successive reversals of the gradient field, and detecting the echo signals so formed.
Preferably the relationship between the excitation pulse and the sinusoidally varying gradient field is such that the gradient field is substantially zero at the instant that the excitation pulse is applied. The excitation pulse is preferably a selective pulse. It can comprise a pulse sequence.
In carrying out the invention additional rf pulses can be applied at the instants of echo peaks. These additional pulses serve to maintain the amplitudes of the echo peaks. The rf phase of the additional pulses are preferably in quadrature to the rf phase of the excitation pulse. The additional pulses may be non-selective.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more fully understood, reference will now be made to the accompanying drawing in which:
FIG. 1 illustrates diagrammatically a waveform of an experiment embodying the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown there in a time dependent magnetic gradient field G of sinusoidal shape. The power requirement to drive gradient coils to produce the sinusoidal waveform G can be reduced by incorporating capacitance and tuning the resulting system to resonance. The cycle time of waveform G is 2.tau. and it passes through zero points at instants .tau. and multiples thereof measured from a start point 0. At time 0, a 90.degree. rf selective excitation pulse P is applied. Pulse P is of an appropriate frequency to excite resonant nuclei in a slice in an object. A free induction decay signal F is generated. The time period over which the signal F is available in solids before it decays away is extremely short. The time period is further reduced by the `dead` time D of the apparatus immediately following pulse P. It is not possible to use the apparatus in time D to detect signal F.
Due to the effect of the repetitive reversals of the gradient field G the initial signal F will be regenerated as a sequence of echo signal E with peaks at the time instant 2.tau. and multiples thereof but with progressively decreasing peak magnitudes.
It will be seen that the first echo E and, of course, subsequent echoes occur well after the end of the dead time D. Furthermore since the 90.degree. pulse P occurs when the amplitude of the
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Journal Magnetic Resonance 29, 1978, see IPER p. 2-pp. 365-366.
Cottrell Stephen P.
Halse Morley R.
Strange John H.
British Technology Group Limited
Tokar Michael J.
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